1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive comprising thermal asperity protection by reducing an electrical bias supplied to a magneto-resistive head.
2. Description of the Prior Art
Disk drives employing the Spin Valve implementation of a Giant Magneto-Resistive head are susceptible to asperities manifesting on the surface of the disk. When the head strikes the asperity, its temperature can increase dramatically which is why the asperity is referred to as a thermal asperity. If the head""s temperature exceeds a threshold known as the xe2x80x9cblocking temperaturexe2x80x9d, the stability of the pinning field is reduced, and as the head cools, the arbitrary magnetic flux emanating from the surface of the disk can permanently change the magnetic orientation of the pinning field rendering the head inoperable.
U.S. Pat. No. 6,069,761 discloses a prior art technique for protecting against thermal asperities by increasing the bias current applied to the head during the thermal asperity event. Increasing the bias current saturates the magnetic field affecting the head so that it is substantially unaffected by the arbitrary magnetic flux emanating from the surface of the disk as the head cools. However, increasing the bias current to saturate the magnetic field further increases the temperature of the head, thereby extending the duration of the thermal asperity event. This is undesirable since the data immediately following the thermal asperity cannot be recovered until the affect of the thermal asperity dissipates. Moreover, increasing the bias current will not protect a giant-magneto-resistive head operating in the xe2x80x9copposexe2x80x9d mode wherein the preferred orientation of the pinning field opposes the bias current field. When a thermal asperity causes the temperature of the head to exceed the blocking temperature, the bias current field may cause the orientation of the pinning field to partially reverse direction until the head cools and the pinning layer magnetic properties become dominate. If the temperature exceeds the blocking temperature for an extended period, the pinning field reversal may be so substantially that it may not reorient to the preferred direction once the head cools. Increasing the bias current during the thermal asperity event, as in the ""761 patent, exacerbates this problem.
There is, therefore, a need to protect against thermal asperities in disk drives employing a magneto-resistive head operating in the oppose mode.
The present invention may be regarded as a disk drive with protection against damage due to a thermal asperity. The disk drive comprises a disk having a surface with an asperity, and a magneto-resistive head actuated radially over the surface of the disk for generating a read signal comprising a thermal asperity component caused by the magneto-resistive head contacting the asperity on the surface of the disk. A bias generator is provided for generating an electrical bias applied to the magneto-resistive head, and a thermal asperity detector detects the thermal asperity component in the read signal. When the thermal asperity component is detected, the electrical bias is reduced for a time interval, thereby protecting the magneto-resistive head.
In one embodiment, when the thermal asperity is detected a timer is started and ran for the duration of the time interval. In one embodiment, the timer comprises a programmable time interval that is optimized relative to the characteristics of the disk drive. In another embodiment, the thermal asperity detector comprises a threshold detector for sensing when a dc component of the read signal exceeds a predetermined threshold, wherein the electrical bias is reduced while the dc component exceeds the predetermined threshold. In one embodiment, the electrical bias is reduced to zero for at least part of the time interval, and in another embodiment a polarity of the electrical bias is reversed for at least part of the time interval.
The present invention may also be regarded as a method for protecting a magneto-resistive head in a disk drive against damage due to a thermal asperity caused by the magneto-resistive head contacting an asperity on a surface of a disk. The magneto-resistive head is actuated radially over the surface of the disk and an electrical bias is supplied to the magneto-resistive head. The magneto-resistive head generates a read signal comprising a thermal asperity component due to the magneto-resistive head contacting the asperity on the surface of the disk. The thermal asperity component in the read signal is detected, and when detected, the electrical bias is reduced for a time interval, thereby protecting the magneto-resistive head.